A carbon dioxide absorber in the form of an absorber cartridge for a rebreathing system is known from DE 10 2004 020 133 B3. The carbon dioxide absorber is fastened to a connection head of the rebreathing system by means of a pivotable mount. The connection head is used to connect the absorber to the rebreathing system or to also replace a used absorber with a new one even during operation. Valves are provided for this within the connection head; on the one hand, these valves bridge over the gas ducts to the absorber in the form of a bypass when the absorber has been removed, so that no gas can escape from the rebreathing system. On the one hand, a gas connection is established to the absorber when the absorber is connected to the connection head. The absorber has, on its top side, a guide plate, which is pushed into the pivotable mount on the connection head. To center the absorber in relation to the connection head, a centering notch is provided at the guide plate and a centering pin on the mount. The absorber is connected to the connection head when the centering pin is in contact with the wedge-shaped centering notch. However, there always is a certain clearance between the pivotable mount and the connection head because of manufacturing tolerances, so that a mismatch may develop between the gas ducts of the connection head and of the absorber. Since the absorber is normally fastened to the underside of the rebreathing system, the centering notch and the centering pin are normally not visible to the user. Thus, the user cannot readily recognize whether the centering pin is located within the centering notch when the absorber is inserted into the mount in a tilted position.